Thermal ink jet printers can provide high speed, and relatively modestly priced printing. First generation ink jet printers were described in the May 1985 issue of the Hewlett-Packard Journal, Vol 36, No. 5. Second generation printers have increased resolution (from 96 to 180 dots per inch) and include color graphics and near letter quality, high-speed text. These printers were described in the August 1988 Hewlett-Packard Journal.
The general principals of operation for the pens of both printer generations are the same. Ink is channeled to chambers containing a thin-film resistor on the floor and a small orifice, or nozzle, on the chamber ceiling. The thin-film resistors are rapidly heated to temperatures exceeding 400.degree. C. The ink directly over an excited resistor within the chamber is vaporized and a bubble is formed. As this vapor bubble grows, momentum is transferred to the ink above the bubble, which causes some ink to be propelled through the orifice and to impact onto paper. Each nozzle fires a drop of about 40-200 picoliters as a result of resistor heating in about 2-4 microseconds.
There are several methods being used to store and deliver ink to the resistor chambers. In one, ink is refilled to the resistor area from an ink reservoir (foam saturated with ink) by capillary action. Other methods use an ink bladder or gravity feed.
Ink in the reservoir forms a meniscus at the orifice on the ceiling when the apparatus is not being fired. In order for ink to flow from the reservoir to the nozzles (that is, the orifices), a priming process using pressure differentials is applied to the pen. There must be a continuous, air-free ink path between the reservoir and the meniscus in each nozzle or else deprinting can occur.
Each single color pen will have a number of nozzles that thus fire a plurality of drops, and the plurality of drops must be properly aligned when they impact on the paper in order to form the characters or graphic design desired. This system is highly dependent upon the ink composition for good performance, and a number of practical considerations are at play in preparing suitable ink formulations, particularly when performing multi-color printing or contemplating "plain paper" uses.
Among the problems encountered is "kogation," which is a coined term unique to thermal ink jet printing, and describes the decomposition of the dye resulting from heating to a high temperature by the hot resistor used to "fire" the bubbles of ink towards the paper substrate. Another problem with ink-jet compositions is evaporation of the carrier fluid so that components in the ink precipitate out and form a plug at the orifice. This can prevent proper firing of the ink and is commonly known as "crusting." To varying degrees of success, attempts have been made to reduce or eliminate the various problems that occur in preparing ink compositions for ink-jet printers.
One group of inks are aqueous based formulations with minor amounts of organic solvent (as cosolvent with water) used to adjust the rate of ink evaporation once on the paper. The solvent system itself is sometimes termed the "vehicle" and can be viewed primarily as a carrier for the dye. Where one desires to use plain paper (that is, paper useful for various purposes within a typical office), then it is typical to use primarily aqueous based inks. For example, plain paper is normally sized with starch designed for water based inks.
U.S. Pat. No. 4,853,037, issued Aug. 1, 1989, inventors Johnson et al., describes an ink composition for printing on plain paper using a thermal ink-jet printer. The ink composition comprises at least one member selected from the group consisting of ethylene glycol and diethylene glycol in an amount from about 5 to about 10 weight percent, a dye from about 1 to about 4 weight percent, and the balance water. Optional components include a biocide and a buffering agent.
U.S. Pat. No. 4,791,165, issued Dec. 13, 1988, inventors Bearess and Norton describes an ink composition for a thermal ink-jet printer comprising about 60 to 90 weight percent water, about 5 to 40 weight percent glycol, about 0.001 to about 10 weight percent polymer blend, and about 1 to about 7 weight percent dye. Suitable glycols are said to include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, 1-methoxy-2-propanol, dithioglycol, and ether derivatives. Examples of specific copolymers include polyvinyl pyrrolidone/polyvinyl acetate copolymers, typically in ratios of 60/40, 70/30.
U.S. Pat. No. 4,789,400, inventor Solodar, issued Dec. 6, 1988, discloses an aqueous based ink jet composition incorporating polymers having a molecular weight average of from about 1000 to 10,000 and said to impart superior drop formation. These compositions have a dye component and optionally a spreading substance in an amount from about 1 weight percent to about 10 weight percent. Specifically such spreading substances are said to be benzyl alcohol, N-butyldiethanolamine, 2-(2-butoxyethoxy)-ethanol, and 1-methyl-2-pyrrolidione. The purpose of these spreading substances is said to increase the spot size on the paper. Although these compositions may be desirable for ink jet printers that are energized by magneto restrictive piezo-electric means (such as disclosed in U.S. Pat. No. 3,846,141), they are not similarly useful in thermal ink jet printers where the ink is exposed to a hot resistor which tends to cause a high molecular weight species as is disclosed to decompose. Such degradation leads to the problems of kogation because carbon forms on the resistor surface and retards heat transfer.
U.S. Pat. No. 4,914,562, inventors Abe et al., issued Apr. 3, 1990, discloses ink compositions for wood-free paper that include between about 5 to about 20 wt. % wetting agent and an organic solvent, such as a primary alcohol, in an amount between about 3 and 30 wt. %. Non-ionic surface active agents and wetting agents are suggested for inclusion into the ink compositions with the non-ionic surfactants in amounts between about 5 and 50 wt. %, more preferably between 10 and 30 wt. %, and with wetting agent in an amount between 9 to 70 wt. %. However, such an ink composition tends to have substantial degradation of print quality due to the lessened amount of water replaced by surfactant and wetting agent, which increases capillarity of the ink.
Another problem in ink jet printing is the formation of bubbles, particularly bubbles in the resistor chambers or in the vicinity of the chamber's orifice. Bubbles can deprime the pen or cause small, deformed drops to be ejected. Conventional defoaming agents have been suggested for inclusion in the ink compositions. For example, U.S. Pat. No. 4,365,035, inventor Zabiak, issued Dec. 21, 1982, discloses inclusion of a defoamer for the purpose of reducing foaming during processing of a jet printing ink in preparation and use. Any conventional defoamer is said to be useful with amounts preferably being 0.002-0.1 wt. %. Specific defoaming agents exemplified are Dow-Corning's DB-31 silicone defoamer, with Foamkill 689 of Crucible Chemical Corporation and Bubble Breaker 748 marketed by Witco Chemical Company being also noted.
Efforts to develop improved ink compositions are continually in progress as ink-jet printers evolve. The present invention is particularly directed to thermal ink jet printers which print in several colors. These pose an additional problem of "bleed", that is, mixing of adjacent colors.